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Community and Q&A

Massachusetts three-decker: energy retrofit during re-siding

cambridgethreedecker | Posted in Plans Review on


We have a three decker In Climate zone 5 with 2×4 walls that we are going to reside with fiber cement. We have 2 different wall cavity insulation types. The first floor has close cell spray foam of about R-20. The second and third floor have “ancient” loose filled blown in cellulose. We plan to have our 2nd and 3rd floor “refilled” with dense pack blown in cellulose. I am concerned about what type of foam panel insulation we should put since we have two types of cavity insulation and need to avoid moisture problems from the first floor closed cell spray foam. Do we have to treat our 1st floor differently than our 2nd/3rd floors. Can we use regular house wrap or does it have to something like “hydrogap”?


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  1. Expert Member
    Michael Maines | | #1

    Alkarim, you didn't state it explicitly but reading between the lines it sounds like you're planning to add a layer of continuous exterior insulation before you re-side? It's not necessary in your climate zone if your goal is just to meet the 2015 IRC building code, but it's a good idea anyway, as long as 27-28% of the total R-value is on the exterior. Zip-R9 on both levels would keep moisture from accumulating. Technically it's best to avoid trapping sheathing between impermeable layers, but with thick enough exterior insulation it shouldn't be a problem.

    The advantage of Zip-R is that is is foam and sheathing all in one package, but instead you could install rigid foam (polyiso is the usual choice) at least 1 1/2" thick with vertical strapping on the outside.

    Including a rain screen or drainage plane behind the siding helps siding dry out, and to keep solar vapor drive from pushing moisture into the sheathing if you use Zip-R. Zip-R properly taped does not require a separate WRB (housewrap) but a separate WRB makes it possible to mechanically lap flashing connections, and is added insurance. Obdyke's Hydrogap provides the extra WRB and a drainage plane in one product. Alternatively, you could use plain old 15# or 30# felt paper, Tyvek, Typar, or one of the high-tech membranes available, covered with vertical strips of wood or plywood to create a ventilated cavity.

  2. cambridgethreedecker | | #2

    Thank you for the response, Sorry for the confusion. I got the impression from another post that I would put a "wrinkled housewrap" directly on the sheathing of three decker. Would that allow me to get a way with a lower exterior foam thickness and/or more permeable foam panel like EPS or XPS. I was hoping to keep the the extra foam to an inch (R-5) thickness. On top of the foam would go vertical strips and then the cement board siding. Would I have to use two different types of foam since the R value of the cell foam is 20 for the first floor (closed cell spray foam ) and the R value of the second/third floor are a R-13 (dense pack cellulose) ? Would there be a need for a second wrap on the foam it self?


  3. Expert Member
    Dana Dorsett | | #3

    Using a crinkle wrap doesn't change the average winter temperature at sheathing, which is a primary factor with interios side moisture drives accumulating over a winter. But it's still a good idea- particularly if you're not re-mounting the window flashing to go to the exterior side of the foam-over.

    Where there's closed cell foam in the cavities the R-value of the exterior foam doesn't much matter. Where there's cellulose in the cavities the R-ratio does matter. If you're limited to an inch of exterior foam use Dow Thermax polyisocyanurate (R7 @ 1") would be sufficient dew point control for full-dimension 2x4 studs w/cellulose (R15.) That would be R22 total, and an R ratio of R7/R22= 32%.

    In MA you may find a lot of the energy retrofit details in this guide useful:

  4. cambridgethreedecker | | #4

    I was wondering? How much additional benefit of the exterior foam in terms of cost? With the blown cellulose (2nd/3rd Floor) at R 15 and the first floor with closed cell spray foam at (R20) I believe I am exceeding code. Would the fiber cement siding be enough to create the thermal break? Would there be any other simpler/cost effective options in creating the missing "thermal break".


  5. GBA Editor
    Martin Holladay | | #5

    Here is a link to an article that explains the minimum thickness requirements for rigid foam installed on the exterior of walls insulated with fluffy insulation (like cellulose): Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "Would the fiber-cement siding be enough to create the thermal break?"

    A. No. Fiber-cement siding doesn't have much R-value.

    Q. "Would there be any other simpler/cost-effective options in creating the missing thermal break?"

    A. It may be hard to show that exterior rigid foam is cost-effective from an energy-saving perspective, but the analysis depends on your time horizon and your expectations for fuel cost inflation. Generally, if you want to create a thermal break on the exterior side of your walls (to address thermal bridging through the studs), the most cost-effective approach is to install used (reclaimed) rigid foam.

  6. Expert Member
    Dana Dorsett | | #6

    R20 closed cell foam in 2x4 construction just meets the letter of code from an R-value point of view, but underperforms a full-depth R20 in 2x6 framing due to the much lower R-of the framing fraction. (The ~3" of thermally wood of the framing is only ~R3.5, compared to ~R6.5 for 5.5" framing).

    From a thermal performance perspective an inch of exterior polyiso and a full cavity fill of open cell foam or cellulose will beat R20 closed cell foam with no exterior insulating sheathing with good measure. The current code minimum for 2x4 construction is R13 cavity insulation + R5 continuous insulation.

  7. cambridgethreedecker | | #7

    Thanks for responses I was hoping to find out how the assembly would workout since a I have closed cell in the first floor cavities. I get the impression from some reading of old posts from here and building sciences that I would have start with a "drainage crinkle wrap on the sheathing that acts as a drainage plane/ building wrap so the sheathing can breath. I would then put the foam panel and I believe I could put the fiber cement directly on the foam board or would I need another layer for crinkle wrap on top of the foam before I put fiber cement. Would the fact the windows are pre-existing affect my choice? Since it is beveled fiber cement lap siding can I avoid having to put furring strips?

    Thanks again,

  8. GBA Editor
    Martin Holladay | | #8

    You are correct that if you stud bays are filled with closed-cell spray foam, and you intend to install a continuous layer of exterior rigid foam, many experts advise that you install a crinkly housewrap (drainage wrap) between the wall sheathing and the rigid foam.

    Concerning whether you can install fiber-cement lap siding over rigid foam without furring strips: contact the siding manufacturer for more information. One such manufacturer, James Hardie, permits the installation of their lap siding directly over rigid foam, but only if the foam is 1 inch thick or less. If the rigid foam is thicker, you need furring strips.

    If you are leaving the existing windows, you have many tricky details to work out. You'll need to figure out how to flash your windows without causing moisture problems, and that is a tricky challenge that is hard to address in a brief answer.

  9. cambridgethreedecker | | #9

    Thanks Martin,

    I was wondering if anyone could point me in the direction of a best practice guide for retrofit exterior foam details that addresses all the details like flashing, jambs, etc. The issues that are being presented to me by the contractor is that the siding will have wavy appearance, window build out will need a historical sill instead of a standard small sill that we prefer and door threshold/jamb extensions.

  10. GBA Editor
    Martin Holladay | | #10

    Al Dhanji,
    Q. "I was wondering if anyone could point me in the direction of a best practice guide for retrofit exterior foam details that addresses all the details like flashing, jambs, etc."

    A. Here is a link to a GBA article on the topic: How to Install Rigid Foam Sheathing. Note that the article includes links to over a dozen other GBA articles on specific aspects of this type of work -- so follow those links, too.

    Here are the two best non-GBA resources on this topic:

    Incorporating Thick Layers of Exterior Rigid Insulation on Walls

    Builder’s Guide to Continuous Insulation

  11. Expert Member
    Dana Dorsett | | #11

    There are quite a few details on insulating sheathing (and other relevant topics covered) in the Mass Save Deep Energy Retrofit builder Guide too:

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